Air sampling pump units having a controlled constant air flow rate are typically used to monitor air to which workers are exposed. These pump units are well known in the art and typical examples of such pumps are disclosed, for example, by Baker et al. in U.S. Pat. Nos. 4,063,824; 4,123,932; 4,269,059; and 4,527,953; by Thomas et al. U.S. Pat. No. 4,384,825; and by Lalin in U.S. Pat. Nos. 4,432,248; 4,532,814; and 4,576,054.
One type of such prior art sampling pump uses a feedback control system based on a pressure switch connected across a variable orifice to measure the air flow rate and vary the pump rate to maintain a constant flow rate. The operator adjusts the variable orifice to set the desired operating flow rate. A separate flow calibration device is used in a trial and error fashion to achieve the desired flow rate.
Another type of such prior art pump uses pump variables such as motor current and RPM to compensate for variations in flow rate. The operator must use a separate flow calibration device and set the sample pump at a flow rate using the same trial and error method as above and then operate the pump at various pressure drops while setting two more variable resistors to set up the compensation circuit for proper functioning at that flow rate. This circuit, when carefully adjusted for proper flow and proper compensation, then compensates to keep flow errors to less than about plus or minus 5%, but at only one flow rate.
Both of these sample pump units lack a means of accurately indicating the pre-adjusted flow rate. Some use a rotameter to indicate flow rate; however they are difficult to read accurately and can be relied upon to produce only plus or minus 20% accuracy.
In view of the drawbacks associated with these conventional air sampling pump units, there is a need for one that can be easily and quickly set to the desired flow rate, that continuously displays in a direct reading format the set flow rate, and that accurately controls flow at that flow rate without further adjustments of the flow compensation circuit, thus eliminating the need for trial and error set-up, thus saving operator set-up time.